Obviously, there are a number of design flaws that can cause an irrigation system to fail – poor installation techniques, using the incorrect equipment, improper scheduling. But improper zoning is right at the top of the list.
Strategically grouping sprinkler heads together allows contractors to take full advantage of the irrigation capabilities of the system and water supply while tailoring the irrigation schedule to meet the varying needs of the different plant material present on the site. Many of the decisions necessary in designing the system will be influenced by the zone setup.
“Thinking about zones starts as early as the contractor’s first meeting with the client,” affirmed Doug Snyder, manager, The Highbridge Corp., Issaquah, Wash. “That’s when you look to select plant material for the job and decide how the site will be graded, both of which have to be decided before you can start to design the system.”
FACT FINDING. Contractors readily agreed that a common mistake in zone setup is designing the system before all of the necessary information has been gathered.
TIP #1: Proper irrigation design requires that the operating pressure within a zone never vary more than 15 percent.
TIP #2: If the elevation difference in a zone is 5 feet, the pressure at the lowest head is 2.17 PSI higher than at the highest head.
TIP #3: Parking lot medians and islands, due to the surrounding asphalt and heat, will require more water and a separate zone.
“Figuring out what you need to know isn’t too difficult for commercial jobs,” related Kevin Smith, president, Computerized Design Services, Longwood, Fla. “I take the total gallonage of all of the sprayheads on the system and multiply that number by 12 minutes run time for the sprayheads. Then I do the same thing for the gallonage for all of the rotors and multiply by 40 minutes run time for the rotors. That gives me a total flow for a day for the entire system.”
Smith’s next step is to figure out a gallon-per-minute flow rate by dividing the total daily flow by the available watering time for a day, which is usually eight hours.
“For commercial jobs, the flow rate should be at least 25 gpm, which isn’t difficult to get,” Smith noted. “The next thing to do is figure out the size of your zones based on the flow rate and then separate the plant material based on irrigation needs.”
“Each site should be divided into areas of different watering requirements such as turf, planting beds, ground covers and so on,” noted Larry Keesen, president, Keesen Water Management, Denver, Colo.
“Zones need to then be scheduled so shrubs are irrigated more deeply and not as often, compared to turf,” commented Smith. “And annuals, because of the challenges of getting them established, need their own zone.”
When compared to commercial systems, residential systems present some different challenges to contractors, according to Brian Quill, irrigation and construction manager at Industrial Landscape, San Jose, Calif.
“The first thing to do is determine the static water pressure, the meter size and the mainline size, and all of that information should be available from the site or the water purveyor,” recommended Quill. “Normally, the home will have an existing meter and line that goes up to the house and a contractor can tap into that line at the hose bib and use a pressure gauge to find the static water pressure.”
Quill was quick to point out, however, that the working pressure must also be determined because the static pressure is only a reading in a ‘no flow’ condition.
“Once the water starts flowing, pressure losses occur from the water flowing through the meter, the supply line and the hose bib,” Quill explained. He also commented that many manufacturers provide resource manuals with pre-calculated pressure losses based on various system components.
“Another valuable piece of information is the available flow,” Quill continued. “That can be determined at the hose bib as well with a 5-gallon bucket and a stopwatch.
“It’s a simple test, but it’s important because you might have a reading of 80 PSI for the system but only get 5 gpm of actual water flow,” he noted. “And the flow on residential systems is usually the primary restriction instead of the pressure because most residences, at least in our part of the country, have a 5/8-inch meter, which is small.”
As a result of restricted flow, it’s common for residential systems to be zoned with many valves since the low flow rate requires one valve for three or four heads.
Quill cautioned against using maximum safety capacities taken from pressure loss tables for designing a system because they don’t fit realistically into system designs.
“Using the maximum capacities just creates water hammer by driving flow velocities way up,” he noted. “Even if the system works, the homeowner will probably hear pipes banging in the house every time the system is turned on.”
Smith also recommended designing a residential system based on 80 percent of the maximum gallonage flowing through the water meter to take into account any water uses that take place inside the house while the system is operating.
“Regardless if it’s a residential or commercial system, zones are important so the system doesn’t exceed the maximum capability of the water supply,” Smith noted. “Exceeding the supply will only result in the system operating at a lower pressure, which creates uneven applications and drives irrigation costs up.”
SETUP PRECAUTIONS. Setting up a system according to flow rates and maximum capacities isn’t necessarily enough to guarantee success, however.
As with the attention required for zoning different plant material, the nature of the property can also demand other design alterations that would influence zones.
Snyder said the first thing he checks for on a new site is its orientation to the sun since plant material on the south and west sides of a property are likely to get more sun and need more irrigation.
“When berms or mounds are present, the top should be watered with a separate zone, even if it consists of only one head,” recommended Keesen. “The peak of the berm will dry out much faster than the slopes, and it will require additional water.”
Snyder agreed with Keesen, and added that a separate zone may need to be included for the middle of the slope, depending on how large the hill is.
|Zone Valve Location|
The most efficient location for the electric control valve is in the middle of the zone, but because of elevation shifts, pressures controls and wire, pipe and trenching costs, this is not always cost effective for the entire irrigation system.
Cost-effective control valve placement allows the valve to be on one side of the zone it serves. If an area is two zones in width, then it would be appropriate to route the main between the two zones. This will save on the cost of pipe and installation, as well as maintain a good balance of pressure throughout the zone.
The same is true at the end of the mainline where it is usually cost effective to stop the mainline prior to entering the last zone or two, depending on the distance and elevation change from the control valve and the closest boundary of the zone.
The mainline route, where the area widens and is more than two zones wide on any side, should have the mainline extended toward that area in order to better control the lateral pressure variation and lateral line surge. (Long straight lines have a much greater potential for surge than do shorter ones, and empty lines from low head drainage will increase the surge damage potential.)
End feeding long, single row zones with 30 to 40 pop-up spray heads can delay by minutes the time it takes for the first and last head on the line to pop up. This affects the water distribution by placing more water closer to the zone. Center feeding the line will reduce surge potential and reduce the time between the first head pop up and the last head pop up.
If there are two or more rows of heads and little slope, shorten the rows and place several rows on the same zone. (Only if all heads have matched precipitation rates.) Heads that are grouped together will cool the air more resulting in less evaporation and better compensation for wind direction changes during the watering cycle.
Valves and valve boxes should be kept away from walks, streets and driveways to avoid damage from vehicles and snow plows, lessen pedestrian liability, reduce visibility and prevent vandalism.
This information was excerpted from The Complete Irrigation Workbook, by Larry Keesen.
“There will probably need to be a zone set up for the bottom of the slope as well, especially if it goes up against a paved surface,” Snyder added. “Concrete or asphalt will create a heat sink effect that will dry out the nearby turf.”
Snyder also commented that contractors can design the most efficient systems in the world, but if the person who will manage the system on a daily basis isn’t trained to understand irrigation needs and how to properly use the controller, the system will fail.
The author is Editor of Lawn & Landscape magazine.
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